[PATCH 4/9] tracing: MIPS: mcount.S: Fix the argument passing of the 32bit support with gcc 4.5

[Wu Zhangjin <wuzhangjin@gmail.com>]

From: Wu Zhangjin <wuzhangjin@gmail.com>

As the doc[1] of gcc-4.5 shows, the -mmcount-ra-address uses register
$12 to transfer the stack offset of the return address to the _mcount
function. in 64bit kernel, $12 is t0, but in 32bit kernel, it is t4, so,
we need to use $12 instead of t0 here to cover the 64bit and 32bit
support.

[1] Gcc doc: MIPS Options
http://gcc.gnu.org/onlinedocs/gcc/MIPS-Options.html

Changes from old revision:
  o Apply the feedback from David Daney:
  define a macro MCOUNT_RA_ADDRESS_REG instead of the magic number $12


Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
---
 arch/mips/kernel/mcount.S |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
index 9a029d4..6bfcb7a 100644
--- a/arch/mips/kernel/mcount.S
+++ b/arch/mips/kernel/mcount.S
@@ -57,6 +57,12 @@
         move ra, AT
        .endm
 
+/*
+ * The -mmcount-ra-address option of gcc 4.5 uses register $12 to pass
+ * the location of the parent's return address.
+ */
+#define MCOUNT_RA_ADDRESS_REG  $12
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 NESTED(ftrace_caller, PT_SIZE, ra)
@@ -70,7 +76,7 @@ _mcount:
 
        MCOUNT_SAVE_REGS
 #ifdef KBUILD_MCOUNT_RA_ADDRESS
-       PTR_S   t0, PT_R12(sp)  /* save location of parent's return address */
+       PTR_S   MCOUNT_RA_ADDRESS_REG, PT_R12(sp)
 #endif
 
        move    a0, ra          /* arg1: self return address */
@@ -142,9 +148,9 @@ NESTED(ftrace_graph_caller, PT_SIZE, ra)
 #ifdef CONFIG_DYNAMIC_FTRACE
        PTR_L   a0, PT_R12(sp)
 #else
-       move    a0, t0
+       move    a0, MCOUNT_RA_ADDRESS_REG
 #endif
-       bnez    a0, 1f          /* non-leaf func: stored in t0 */
+       bnez    a0, 1f  /* non-leaf func: stored in MCOUNT_RA_ADDRESS_REG */
         nop
 #endif
        PTR_LA  a0, PT_R1(sp)   /* leaf func: the location in current stack */
-- 
1.7.0.4